Such a drying method is already known, whereby, in order to dry the compressed gas coming from the compressor device, a part of the gas is first cooled in a cooler and then sent through the drying pressure vessel and whereby, in order to regenerate the other pressure vessel, the non-cooled part of the gas is guided through the regenerating pressure vessel so as to absorb the adsorbed moisture from the drying means by means of the compression heat stored in this part of the gas to be dried.
A disadvantage of the known methods for drying compressed gas is that the pressure dew point of the dried compressed gas at the outlet of the dryer, depending on the working conditions of the dryer and the compressor device, can be relatively high.
Thus, with the known methods, in an environment with a low air humidity, better results are often obtained than in hot, damp environments.
In order to remedy this, it is known to additionally heat the part of non-cooled gas which is used to regenerate the drying agent by means of a heating element provided to that end, such as an electric heating element, to thus improve the dew point.
Since only the gas part needs to be heated which is used to regenerate one of the pressure vessels, a small and thus cheap and compact heating element will do, and thus only relatively little power is required to heat said limited gas stream.
As the heating power of the heating element is limited, one must pay attention that the volume of gas which is sent to this heating element to regenerate the regenerating pressure vessel is not too large. For, when this partial flow to the heating element becomes too high, the aimed temperature rise of the gas stream cannot be reached, as a result of which the dryer can no longer function as required.
Moreover, restricting this non-cooled gas stream to the heating element is advantageous in that use can be made of smaller pipe diameters and valves, such that a cheaper installation can be provided to implement the drying method.
In order to adjust the flow of the non-cooled part of compressed gas flowing to the heating element, use is made according to the known drying methods of a flow measurement of the gas stream to the heating element, and the volume of non-cooled gas that is supplied from the gas stream to be dried, entering the dryer, is adjusted on the basis of the above-mentioned flow measurement carried out by means of a flow meter provided to that end in the line to the heating element according to the known method.
A major factor that made such a flow measurement necessary up to now, is that a minimal gas stream must always be provided over the heating element, in order to make sure that:                the heating element cannot become damaged due to overheating; and that        a correct temperature measurement of the gas is made possible, since the temperature to be measured (which is decisive for the pressure dew point at the outlet of the dryer) must be transferred to a temperature sensor provided to that end, which requires such a minimal gas stream.        
Applying a flow measurement by means of a flow meter is disadvantageous, however, in that:                such a flow meter is expensive,        a stable gas flow is required at the flow meter in order to be able to carry out a precise measurement, to which end a minimal length of straight line must be provided, upstream and downstream of the flow meter, which has a negative effect on the dimensions and price of the dryer and moreover results in that the dryer's construction strongly differs from the embodiments without a heating element, as a result of which parts are not interchangeable and thus must be manufactured in smaller numbers, which is relatively expensive;        a pressure drop and consequently a loss of energy occurs when the gas stream flows over or through the flow meter;        the reliability of the dryer entirely depends on the good working order of the flow meter; and        when the partial flow to the heating element is kept constant, an adjustment will be required to switch said heating element on and off in order to prevent the temperature from deviating too much from the set value. This has for a result that the available heating power is not optimally used and that the heating step must be prolonged somewhat more, such that more drying agent must be provided, as a result of which it takes longer to regenerate an amount of drying agent.        